An engine may burn gaseous fuels such as hydrogen (H2), natural gas, lighter flammable hydrocarbon derivatives, etc. Using gaseous fuel in an internal combustion engine can provide various advantages compared to liquid fuel injection, such as reduction of exhaust emissions of nitrogen oxides, particulate matter, and hydrocarbons.
In one approach, a pre-mixed approach (or external mixture formation), which may include port injection, is used to induct gaseous H2. This method may result in reduced volumetric efficiency by a factor of up to thirty percent at stoichiometry due to the displacement of air by the gaseous H2, and a consequent loss in torque and maximum power. Also in these systems, the H2 may be exposed to hot residual gases as well as heated intake manifold and cylinder head porting and cylinder walls during the intake stroke, which can causes a propensity to pre-ignition and further reduction in engine output. Further, such effects can be exacerbated if the H2 injection duration is extended.
One approach to address the issues of pre-mixed delivery is to directly inject the gaseous fuel into the combustion chamber. Specifically, JP 63-198,762 describes a method of directly injecting H2 into a combustion chamber. In the '762 reference, it appears that the exhaust valve is closed very near, if not at, top dead center of piston position when an exhaust stroke is completed, and the hydrogen injection valve is opened immediately after this exhaust valve is closed to inject hydrogen gas into a combustion chamber. Next, after the hydrogen injection valve is closed at the rotation position of the crank angle of about 90 degrees, the suction (intake) valve is opened, and supercharged air is fed to the combustion chamber and mixed with hydrogen. Then, the mixture is compressed and ignited.
However, the inventors herein have recognized several disadvantages of such an approach. For example, when the exhaust valve is closed, the pressure in the combustion chamber may be substantially lower than the injection pressure. Thus, the injected gaseous fuel may undergo a sudden expansion and result in flow losses upon entering the combustion chamber. Further, because cylinder pressure may be reduced during the first part of the intake stroke (before the intake valve is opened), increased pumping work may result. Consequently, any injection energy of the gaseous fuel may not be efficiently utilized. Moreover, a propensity to pre-ignition may still exist at increased load conditions.
In one embodiment, at least some of the above issues may be addressed by a method for an engine capable of burning gaseous fuel, the engine also including a combustion chamber, at least one intake valve, and at least one exhaust valve, an injector to directly inject gaseous fuel into the combustion chamber, and a variable valve timing system coupled to the intake valves and exhaust valves. The method comprises of closing the exhaust valve before top dead center of piston position to increase combustion chamber pressure achieved at top dead center and to trap exhaust gas in the cylinder; and starting injection of a gaseous fuel directly into the combustion chamber after the exhaust valve is closed and before the intake valve is opened.
In this way, it is possible to increase cylinder pressure into which the gaseous fuel is injected, and thereby reduce expansion flow losses, and better utilize the fuel pressure to perform expansion work during the cycle. In one particular example, the exhaust valve closing timing can be varied with operating conditions so that a desired cylinder pressure into which injection occurs can be achieved. In another example, the timing of the intake valve may be adjusted later and varied with operating conditions to reduce any flow of fuel or cylinder contents into the intake manifold.
In other words, by injection of a gaseous fuel near the top dead center of negative valve overlap, it is possible to utilize the injection pressure energy of the gaseous fuel to a greater extent. For example, early exhaust valve closure can elevate the pressure in the combustion chamber at top dead center so that the injected gaseous fuel undergoes less sudden (or dramatic) expansion and thus with less resultant flow losses upon entering the combustion chamber.
Further, the combination of early exhaust valve closure and late intake valve opening can have only a small overall effect on the pumping work. Thus, the injection of gaseous fuel can produce more useful work than that produced by variable intake valve timing alone. Furthermore, in one example, it is possible to reduce a propensity to pre-ignition since by closing the exhaust valve early while enriching the overall air-fuel ratio as load increases, less oxygen is available to cause pre-ignition because residual gases from the previous cycle are left in the combustion chamber. Thus, a propensity of pre-ignition can be reduced even in the conditions where such pre-ignition is most likely to.
According to another aspect, a method is provided for an engine capable of burning gaseous fuel, the engine also including a combustion chamber, at least one intake valve, and at least one exhaust valve, an injector to directly inject gaseous fuel into the combustion chamber, and a variable valve timing system coupled to the intake valves and exhaust valves. The method comprises of adjusting exhaust valve closing timing and keeping intake valve closed to increase a pressure in the combustion chamber approximately equals an injection pressure of gaseous fuel when a piston is at top dead center; injecting gaseous fuel; and opening the intake valve when the pressure in the combustion chamber approximately equals or is less than manifold absolute pressure.
Again, such operation can provide several advantages, such as improved utilization of fuel injection pressure, reduced tendency to pre-ignition, and improved fuel economy.
Note that various intake and exhaust valve configurations may be used, such as electrically actuated valves, variable valve timing, multiple intake valves, multiple exhaust valves, and combinations thereof.